Thunderstorm observation satellite

ABSTRACT

This relates to equipment on board a spin-stabilized satellite for locating and counting lightning discharges. The discharges are detected by a plurality of n X n photocell matrices to deliver two quantized angles which define unambiguously the position of the discharge projected on the surface of the earth. The matrices effectively divide the earth surface into 2H X 2H km. zones (H being equal to the altitude of the satellite) centered along the subsatellite great circle and each zone is quantized into n X n quadrilaterals. The angle values are processed to obtain coordinate values defining the quadrilateral where the discharges occurred. The coordinate value are stored in the appropriate one of n X n portions of the appropriate one of a plurality of zone memory. Further processing allows discrimination between long and short discharges which information is stored in the appropriate one of the n X n portions of the proper memory during the time the quadrilateral corresponding to the memory portion was observed by the satellite. The content of the memories are sent to ground stations at regular intervals.

Turriere [54] THUNDERSTORM OBSERVATION SATELLITE [72] Inventor: Jean Emile Turriere, Paris, France [73] Assignee: international Standard Electric Corporation, New York, NY.

22 Filed: Feb. 19,1970

211 Appl.No.: 12,791

Related US. Application Data [63] Continuation-impart of Ser. No. 712,102, Mar. 11,

1968, abandoned.

[30] Foreign Application Priority Data Mar. 31, 1967 France ..l009l4 [52] US. Cl ..244/1 SS, 250/220 M [51] Int. Cl ..B64g 1/00 [58] Field of Search ..244/1 SS, 77, 1 SA; 250/209,

B. N. Harden & V. A. W. Harrison; Satellite-borne Equipment for a Global Survey of Terrestrial Radio Noise; The Radio & Electronic Engineer; Mar. 1968; pp. I25- I33 R. A. Hand & D. Q. Wark; Physical Measurements from Meteorlogical Satellites, Astronautics & Aerospace Engineering; Apr. 1963; 85- 88.

Primary ExaminerTrygve M. Blix Assistant Examiner-Jeffrey L. Forman Attorney-Cv Cornell Remsen, Jr., Walter J. Baum, Paul W. Hemminger, Charles L. Johnson, Jr., Philip M. Bolton, Isidore Togut, Edward Goldberg and Menotti U. Lombardi, Jr.

[57] ABSTRACT This relates to equipment on board a spin-stabilized satellite for locating and counting lightning discharges. The discharges are detected by a plurality of mm photocell matrices to deliver two quantized angles which define unambiguously the position of the discharge projected on the surface of the earth. The

matrices effectively divide the earth surface into 2HX2H km. zones (H being equal to the altitude of the satellite) centered along the subsatellite great circle and each zone is quantized into nXn quadrilaterals. The angle values are processed to obtain coordinate values defining the quadrilateral where the discharges occurred. The coordinate value are stored in the appropriate one of nXn portions of the appropriate one of a plurality of zone memory. Further processing allows discrimination between long and short discharges which information is stored in the appropriate one of the nXn portions of the proper memory during the time the quadrilateral corresponding to the memory portion was observed by the satellite. The content of the memories are sent to ground stations at regular intervals.

7 Claims, 36 Drawing Figures PATENTED FEB 1 1972 saw 020F 13 T I Inventor JEAN EMILE TURR/ERE PATENTED FEB 1 1972 SHEET 03 0F 13 Inventor E fMl E TURR lERE y WQ.

Agent Inventor JEAN EMILE TURR/ERE PATENTED FEB 1 1312 saw 10 0F 13 QINNMYMVMN wl wkhm JEAN EMILE TURR/E/Qi Pmmmm 18:2 3,638,882 SHEET 12UF i3 Tit/82C! INPUT CONTROL CIRCUIT ICC REGISTER nzq/srm o I w wpar/our arl l REG/875R i L88 v I J QMTB sa MEMORY I $U8TRACTR f8. m 787. C2

t5; I07(C2fC4+C6) OUTPUT CONTROL CIRCUIT INVEN FOR JEAN 6. TURR/RE BY QWQW AGENT THUNDERSTORM OBSERVATION SATELLITE CROSS-REFERENCE TO RELATED APPLICATION this is a continuation-in-part of copending application, Ser. No. 712,102, filed Mar. ll, l968, and now abandoned.

BACKGROUND OF THE INVENTION The present invention relates to a system for detecting, localizing and measuring the light phenomena occurring in the troposphere or on the earth surface with the help of a satellite wherein the observation is carried out by optoelectronic transducers constituted by photosensitive means, such as photoelectric cells.

Many processes are known to observe such phenomena and more specially to observe lightning occurring during thunderstorms. These processes can be classified into five categones:

l. Observation in the very low frequency band (I to 30 kl-Iz.): the atmospherics" produced by the lightning have been studied for a long time by several specialized stations equipped with radiogoniometers, but their localization is obviously difficult. The atmospherics" can also be detected by receiving stations placed in satellites, but their localization is not possible because of the peculiar conditions of propagation of the myriametric waves in the ionosphere.

2. Observation in the high very high frequency band (100 kHz. to several megahertz). Experiences of reception on earth in these frequency bands have been done (Book: Physics of Lightning page l34-Author: D. J. Malan-Editor: The English University Press).

The very high frequency waves can also be received in satellites equipped with antennas designed for these frequencies. (Publication Lightning Observations on SatellitesAuthor: A. S. Dennis-Stanford Research InstituteContract NASr 49 (18).) However, it is understood that such an antenna has an important size and must be permanently directed towards the earth.

3. Observation on a radar screen: By observing thunderstorms with the help of radar operating in the UHF band, real echoes can be obtained on the ionized trace of the lightning (Book: Handbook of GeophysicsAuthor: S. L. ValleyEditor: Mac Grew Hill-Apr. 1965). This process allows to localize the lightning, but the range is limited to that of the radar and important video equipments and are needed if any information is not to be lost.

4. Photographic observation: Thunderstorms and lightning can be photographed intermittently from the earth surface or from balloons, but the range is limited to a few tens of kilometers (Book: Physics of Lightning" hereabove mentioned page 9). This observation process is reserved to the spectral measurement of lightning (Article: Lightning Spectroscopy- -Author: M. A. Uman-IEEE SpectrumAug. l966page 102).

5. Observation from the earth surface with the help of photoelectric cell or a photomultiplicator tube provided with a narrow band optical filtering (Article: Lightning Photography and Counting in Daylight-Authors: L. Salanave and B. Brook-Joumal of Geophysical Research-Mar. l5, l965-page L285). This process presents the drawback that this narrow band filtering is really efficient only for a very reduced sight field, and that it can allow neither the observation nor the localization of lightning occurring in a large zone.

SUMMARY OF THE INVENTION In the process according to the invention, the observation is made in an optical way with the help of optical-electrical transducers placed in a satellite, the filtering of the ambient light and of the other causes of interference being preferently carried out by electronic means.

The satellite, placed at a medium altitude, on a circular or quasicircular orbit is spin-stabilized and rotates around an axis perpendicular to the plane of the orbit. It is equipped with four plane mosaics" each comprising nXn photoelectic cells. The

aperture of the optical system associated with each mosaic is chosen in order to obtain sight angles of 90 in each direction. These mosaics are used to make a quantized measurement of the parameters necessary to localize the observed phenomena.

If the satellite is at a height H, a mosaic perpendicular to the vertical of the subsatellite point covers, on the tangent plane to the earth, a square surface with a side equal to 2H. Because of the curvature of the earth, the corresponding zone on the earth has a greater surface. It has been chosen, as an example, to quantize the earth surface in quadrilaterals each side of which having a length equal to 2H/n, that is, each zone of the earth is quantized, as the mosaic, into nXn quadrilaterals. Each of said quadrilaterals is localized, with the help of exact geographic coordinates, with respect to a reference quadrilateral centered on a point of the zone flown over by the satellite that will be called the origin point. Thisorigin point is the center of the central quadrilateral of the zone if n is an odd number.

This localization concerns the oblique projection, on the earth surface, of the phenomena in the troposphere, the satellite being taken as the pole of the projection. For light phenomena, such as lightning generated by thunderstorms, which occur at relatively low altitudes with respect to the altitude of the satellite, the above-mentioned oblique projection can be taken as the projection according to the vertical of the I place, and in the contrary case, it would be possible to transcoordinates of the earth quadrilateral containing this point with respect to the origin point. This information is then temporarily stored in a buffer memory and compared to the information received during a given period of time. From this comparison there is generated information on the duration of the light phenomena. (For example: long lightning En or short lightning Ec.)

In addition, there is placed in the satellite, 2 zone memories each comprising nXn=n addresses corresponding to the quadrilaterals of a zone, plus an address in which is written information characterizing the position of the origin point with respect to an earth reference point. If this information is set up for each passage over this point, it enables to obtain afterwards, by consulting the satellite orbiting information table, the exact coordinates of each origin point. Each address of a quadrilateral, selected from the computed coordinates, may storein the case of the observation of thunderstormstwo pieces of information characterizing, respectively, the number of short duration lightning flashes and the number of long duration lightning flashes observed in this quadrilateral.

Then, at regular intervals, the contents of each zone memory is transmitted to ground where it is received, for example, by the network of APT (Automatic Picture Transmission) stations.

It is to be understood that this optical surveillance process allows the measurement of other parameters, such as, for example, the peak amplitude of the light pulses at given points (or at every point) covered by the mosaic.

In the process according to the invention, the localization of an earth point is made by a spin-stabilized satellite equipped with four photoelectric mosaics placed on i s periphery. It is quite obvious that this operation can also be made with a Nimbus" type satellite stabilized along its three axis and comprising only one mosaic that is permanently directed towards the earth. However, the stabilization of such a satellite is a difficult operation and must be frequently corrected, which entails having fuel reserves on board.

Thus, the object of the present invention is to localize the light phenomena occurring in the troposphere or on the earth surface, and to measure some of their characteristics, the detection means being placed in a satellite.

The invention is characterized by the fact that the detection means of the light phenomena is constituted by four mosaics each equipped with n n photoelectric cells regularly placed on the circumference of the satellite in the case where said satellite is stabilized by a rotary motion around an axis perpendicular to the plane of the orbit; that an optical system with an aperture of 90 is associated to each mosaic, said mosaic being placed in the focal plane of said optical system; and that the shape of the cells is such that each one represents an angle quantum having a value of 90/n, said mosaic allowing the measurement of the direct sight angle 8] and of the lateral sight angle Ax belonging to the direct sight triangle drawn from the center of the satellite. This triangle is perpendicular to the orbit plane and comprises the line of sight, the angle Ax being the apex of this triangle and the angle Bj being the angle made by the plane of this triangle with the plane perpendicular to the orbit plane. These angles being measured in a cartesian coordinate system Fx], F yI where F is the focus of an optical system associated with a mosaic.

Another characteristic of the present invention is the fact that, when a cell of the mosaic of coordinates x y detects a light phenomena in its sight range, information is registered in two flip-flops corresponding to the ordinate and to the abscissa of this cell, one of said flip-flops belonging to a lateral angle register and the other of said flip-flops belonging to a longitudinal angle register; and that this information is encoded to obtain the codes CAx and CBj of these angles expressed in an angle unit, In addition, a horizon sensor delivers, at any time, the code CB of the angle between the vertical and a reference point on the satellite surface; and that the angle value measured by the mosaic is corrected to obtain the real value By=BjBo.

Another characteristic of the present invention is the fact that, if the great circle CF which is orthogonal to the great subsatellite circle CS and which passes through the point E where the observed phenomenon has appeared is considered, the intersection point of these two circleswhich is placed between the equatorial plane and the satellite-being referenced E, a point is defined on the earth surface by its curvilinear ordinate Y measured, with respect to an origin point, on the circle CS and by its curvilinear abscissa X that represents the length of the arc EE; that the measure of the angles Ax, Bj, B0 is made in a discontinuous way under the control of a signal 1A having a period of t milliseconds and a duty cycle lower than I; and that, during the remaining part of the period, or during the next period, there is calculated, with the parameters Ax, Bj, B0, H and R, the coordinates X and Y, where H is the height of the satellite and R is the earth radius. The phase duty cycle lower than 1 means that the duration of the signal 1A is shorter than the duration of its repetition period. For instance, a square wave (equal on and oh durations) has a duty cycle of 0.5.

Another characteristic of the invention is the fact that the earth surface is divided into zones of 2H 2H km. (kilometers); that each zone is quantized into nXn quadrilaterals, n being an odd number; that each quadrilateral is identified in cartesian curvilinear coordinates with respect to the center of the central quadrilateral or origin point; that a zone covers a surface smaller than that covered by a mosaic parallel to the plane tangent to the origin point; and that the coordinates Y and X expressed in length units are transformed into coordinates Y and X characterizing, in quantized units, the quadrilateral in which the phenomenon has appeared.

Another characteristic of the invention is the fact that the codes CX, CY of the first light discharge sensed in this quadrilateral are stored on a line of a first cyclic memory; that, at each period of duration 1, all the codes stored in this memory are compared to the codes of a discharge sensed during the preceding period; that, when there has been detected a light phenomenon, such as a short lightning (associated signal Be), or a long lightning (associated signal En), its codes CX, CY are written asynchronously in a second cyclic memory; that, when the codes are read in said second memory during the cyclic readout, this readout is blocked, when signal Ec or En is read; that the associated code CY is examinatcd to know if its value is lower or higher than (12-1 )/2; that in this last case, there is calculated Y(nl/2)=Y0; and that if Yo (nl/2), the information X and Y0 is discarded.

Another characteristic of the present invention is the fact that there are Z zone memories selected in time succession in synchronism with the displacement of the satellite; that each memory comprises n addresses selected by the codes CX and CY (or CYo), plus one address for registering a code CZ characterizing the number of zones flown over by the satellite from a reference point on the earth surface, each of the 11 addresses comprising two parts for storing, respectively, the number of short and of long lightning sensed in the quadrilateral; that, at a given time, a high-stability clock delivers, first, the code CZ and, second, cyclic selection signals for the Z zone memories; that, if Y (nl/2), the selection signal j, present at that time, controls the selection of the memory j, whereas, if Y (n-l/2), it is the memoryj+l that is selected; and that the readout of the memory j, for example, and the transmission of the information to the ground are carried on when the writing is finished in the memory j1 Another characteristic of the invention is the fact that, when the orbit makes an angle greater than 10 with respect to the equator, a supplementary correction is made on the coordinates X and Y for obtaining the true coordinates X and Y, said correction taking into account the earth rotation.

BRIEF DESCRIPTION OF THE DRAWING Other objects, characteristics and advantages of the present invention will become apparent by reference to the following description taken in conjunction with the accompanying drawings, in which:

FIGS. Ia to 1m represent symbols of logical circuits used in FIGS, 9, 11, l2, l5, l6, l7 and 18;

FIG. 2 represents the characteristics of the light discharges and of lightning;

FIG. 3 represents a section of the satellite;

FIG. 4 represents a first perspective view of the earth and of the satellite;

FIG. 5 represents a second perspective view of the earth and of the satellite;

FIG. 6 represents the mode of division of the cells of a mosaic;

FIG. 7 is a block diagram of the electronic circuits in the satellite;

FIGS. 81:, 8b, represent diagrams relative to elementary time signals and to the operation signals;

FIG. 9 is a detailed block diagram ofcircuit ND and mosaics PS of FIG. 7;

FIG. 10 illustrates the organization of memory MC of FIG. 1 1;

FIGS. 11 and 12 are detailed block diagrams of the discharge counting circuit;

FIGS. 13a to I32 represent a given number of diagrams relative to the writing operation in the zone memories;

FIG. 14 represents the organigram of the ordinate correction;

FIG. 15 represents the detailed diagram of the clock CS;

FIG. 16 is a block diagram of zone memories MZ of FIG. 7;

FIG. 17 is a detailed block diagram of one possible embodiment of computer GC of FIG. 7; and

FIG. 18 is a detailed block diagram of memory MTB of FIG. 17.

DESCRIPTION OF THE PREFERRED EMBODIMENT Before describing the invention, there will be a brief discussion of the logic algebra notations which will be used herein in order to simplify the description of the logic operations. The subject has been treated extensively in numerous papers and in particular in the book Logical Design of Digital Computers by M. Phister (J. Wiley-publisher).

Thus, if a condition characterized by the presence of a signal is written A, the condition characterized by the absence of the said signal will be written Z.

These two conditions are linked by the well-known logical relation A Z=0, in which the sign x is the symbol of the coincidence logic function or AND function.

If a condition C appears only when conditions A and B are simultaneously present, then AXB=C and this function may be carried out by means of a coincidence or AND circuit.

If a condition C appears when at least one of two conditions E and F is present, then E+F=C and this function is carried out by means of a mixing gate or OR circuit.

Since these AND and OR logical functions are commutative, associative and distributive, the following relations may be written:

Referring to FIGS. la to Im, the symbols used in the drawings will be described:

FIG. 1a represents a simple AND circuit.

FIG. lb represents a simple OR circuit.

FIG. 1c represents a multiple AND circuit, which comprises, in the case of the example, four AND circuits each having a first input terminal connected to each one of the conductors 91a and a second input terminal connected to a common conductor 91b.

FIG. 1d represents a multiple OR circuit which comprises, in the case of the example, four OR circuits each having two input terminals 91c and 91d, and which delivers, over the four output conductors 9lle, the same signals as those applied over either of said input terminals.

FIG. 16 represents an AND circuit having two input terminals 91f, 91g and which is blocked when a signal is applied over the input 91f.

FIG. 1f represents a bistable circuit or flip-flop to which a control signal is applied over one of its input terminals 921 or 924) in order to set it in the I state or to reset it in the state. A voltage of same polarity as that of the control signal is present, either on the output 93-1 when the flip-flop is in the I state, or on the output 934) when it is in the 0 state. If the flipflop is referenced B1, the logical condition which characterizes the fact that it is in the 1 state will be written B1 and that haracterizing the fact that it is in the 0 state will be written Bl.

FIG. lg represents a group of several conductors, five in the considered example. In some cases, when such a group must transmit the l and 0 informations originating, for example, from a register, the number of conductors is followed by: (1-0).

FIG. 1h represents a flip-flop register. In the case of the figure, it comprises four flip-flops having the 1 input terminals connected to the conductors of the group 92a and the I output terminals connected to the conductors of the group 93a. The digit 0, placed at one end of the register, means that this latter is cleared when a signal is applied on the conductor 91h. The bracketed digit which is placed inside the symbol indicates the capacity of the register.

FIG 1i represents a decoder which, in the case of the example, transforms a four-digit binary code group applied over the group of conductors 94a into a l-out-of-l6 codes, so that a signal appears on only one among the 16 conductors 94b for each one of the code groups applied at the input.

FIG. lj represents a decoder which is so designed that it delivers an output signal only when the binary code group corresponding to the decimal number is applied over its input terminals.

FIG. llk represents a flip-flop counter which counts the pulses applied to its input terminal 940 and which is cleared by the application of a signal on its input 94d. The l outputs of the flip-flops are connected to the output conductors 94c.

FIG. 1C represents a selector constituted by the combination of a counter and of a decoder such as shown in FIG. Ik and 11'.

Im represents a code comparator which delivers a signal over its output terminal 95a when the three-digit code groups applied over its terminals 95b and 95c are identicalare not identical.

In the course of the description frequently the reference character of a signal will be preceded by the letter C for identifying the binary code which, when decoded, gives said signal. Thus, CWx" designates the code corresponding to the signal Wx.

It should be noted that, in the different figures associated with the description, the electronic gates (AND, OR circuits) are not referenced. In fact, every gate is unambiguously identified, in the text, by the logical equation describing the function it performs and by the figure number, the reference of each applied elementary signal being placed, on said figure, near the corresponding input terminal. Thus, the AND circuit of FIG. 1a is defined by writing the logical equation WtXWu followed by the number of the figure.

There will be described, by way of nonlimitative example, a system for localizing light phenomena specially adapted to the counting of the lightning flashes occurring during thunderstorms.

As it is well known, each lightning is composed of one or several light discharges, the average characteristics of which are represented in FIG. 2. For each light discharge, its rise time tr is equal to at least 5 psec. (microseconds) and, on the I average, 20 psec.; its fall time tf at 6db. (decibel) is equal to,

on the average, 100 pace. and, at its maximum, a few milliseconds (msec.); and its repetition period TR is between some tens to some hundreds of milliseconds. A succession of discharges coming from the same zone of the troposphere characterizes a lightning and comprises as many as 20 discharges (four on an average) having a maximum duration of 500 msec. (10 percent of the lightning having a duration higher than 500 msec.).

The detected lightning has herein been classified as short lightning characterized by the logical condition Ec, or as long lightning characterized by the logical condition En.

If To is called the time of occurrence of a first discharge in a zone, then, by way of a nonlimitative example, the following discrimination algorithm is employed: this discharge belongs to a short lightning if no new discharge appears in this zone in the time interval (T0+Tp) to (To+Tq) and to a long lightning in the contrary case (it will be seen further on that it has been assumed that Tp=l 28 msec. and Tq=5 l 2 msec.

In the system for localizing light phenomena according to the invention, the sensing is made by photoelectric mosaics placed in the satellite which is spin-stabilized with respect to an axis perpendicular to the orbit plane.

FIG. 3 represents a section of the satellite ST in a plane perpendicular to its rotation axis I. This figure shows four mosaics PS1 to PS4 regularly placed around the axis I of satellite ST. P associated with each of the mosaics is an aiming optical system P11, P12, PJ3, PM having an aperture of so that, for an altitude H, each mosaic, placed at the focus of the associated optics, Covers on the ground a zone having a side greater than 2H.

Each of these mosaics is divided into n cells which enables the quantized measurement of two angles defining unambiguously the position of a point where a light phenomenon is detected.

This division, that follows particular laws as it will be pointed out hereinbelow, is carried out according to two rectangular coordinate axises and if, for example, n=25, each cell covers a sight field of 90/25=3.6.

If it is assumed that the cells have a response from the near ultraviolet (about 0.41:. (micron) to the near infrared I l p. they can be excited by many phenomena different from the light discharges corresponding to the lightning. Particularly, under the effect of daylight, the edge of a cloud or an earth-tosea transition provokes important variations of the light level received by a cell.

Thus, if the satellite has a rotation of 60 per second (10 revolutions per minute) and for a sight field of 3.6, a cell scans the edge of a cloud in 60 msec. producing, in daylight, a variations of its output signal. In the most unfavorable case, the rise (or fall) time of this signal is 60 msec., which means a fundamental frequency of l6.6 l-Iz.

Thus, it can be seen that by eliminating the low-frequency components of the signal spectrum, (for example, the frequencies lower than 250 Hz.) these stray signals are suppressed without modifying the signals having a short rise and fall times which characterize the discharges (see FIG. 2). From the slope of these rise and fall times, the high limit of the superior frequency band may be set at about 25 kHz.

The noise originating from the ambient brightness in the illuminated hemisphere also produces a stray signal the level of which varies, on one hand, because of the clouds and of the discontinuities and, on the other hand, because of the rotation of the satellite. A great part of the spectrum of this noise is eliminated by filtering the low frequencies, and an optical filter can also be added.

Concerning the design of the mosaic, very many types can be considered and, particularly, those that use thin films of semiconducting materials, silicon, for example, constituting photodiodes, the common cathodes and anodes of which represent the lines and the columns of the mosaic, respectively. These films are disposed on sheets with technological processes which are well known today.

Each of these lines and columns is connected to one terminal of a resistor, the other terminal of said resistor being grounded so that the currents delivered by the photodiodes are represented by voltages proportional to the power of the light signals.

FIG. 4 represents in perspective view the earth FR and the satellites ST after choosing, as an observation place, a point situated in space, on the right-hand portion of the figure.

The satellite ST is represented in a schematic way as a wheel and the lens PJ 1 is shown on the periphery of the satellite. The vertical I passing through the center I of the satellite and the center 0 of the earth defines the subsatellite point Si, the trajectory of which is the great circle CS on the earth surface, the earth being temporarily assumed motionless to make the description easier.

The orbit plane P0 is defined, on either side of the point S], as the vertical plane, the intersection of which with the earth is the great circle CS.

The normal plane PN is defined as the vertical plane that is orthogonal to the orbit plane and which contains, on one hand, the vertical I 0 and, on the other hand, the rotation axis U 1, U2 of the satellite ST. This plane intersects the earth surface according to a great circle CN.

The point E represents the projection on the earth of a point of the troposphere where a light phenomenon appears, and the points Sj and E are on the same great circle CD.

Inasmuch as the satellite is at a known distance H from the earth, it can be assumed, first, that, as it has been seen previously, the origin point of the light phenomenon and its projection on the earth at point E are coincident and, second, that the lens associated with each mosaic, such as PJ 1, is placed at the axis 1 of the satellite. There can then be defined the followlhe line ofsight IE; and

The direct sight triangle, the part of which exterior to the earth surface is the triangle EIE", this triangle being drawn so that its plane is perpendicular to the orbit plane P0. This triangle is perfectly defined by the direct sight angle Bj between its plane and the normal plane and by the lateral sight angle Ax which is the apex angle I. Inversely, knowing these two angles enables the location of the position of the point E on the earth, it being understood that this point is situated between the equatorial plane (the horizontal plane passing through the center 0 of the earth) and the tangent plane at point Sj; and that, the distance ISj (altitude H of the satellite) is known and practically constant (satellite on a circular orbit).

In the satellite ST, there is defined a system of cartesian coordinates IXl, 1Y1, ill, the direction cosines of which are directed as shown in FIG. 4. It results that, in the case shown in FIG. 4, in which the point E is situated in front ofFIG. 4, the angle Ax is positive and the angle Bj is negative.

The quantized measurement of these angles Ax and Bj is made by means of the mosaics which are so divided that the division Kij corresponds to a line of sight of quantized angular coordinates (Ax)i, (Bj)j. Each mosaic is a plane and, using its own system of coordinates Fx, Fy (FIG. 6), it is divided into n divisions along each axis, so that each division covers a sight field of /n for each value (Ax)i, (Bj)j.

To determine the quantizing laws of a mosaic assumed to be horizontal, refer to FIG. 5 which is a perspective view of the satellite and of the earth showing one of the dihedrals formed by the planes PN and PO, the intersection of said planes being the vertical line 15].

In the cartesian coordinate system IX 1 I Y], 121, the direct sight triangle is the hatched triangle ele perpendicular to the plane P0 with an apex angle I having a value Ax, the angle Bj being measured in the plane P0.

Consider first that the angle Ax is constant and that there must be located the locus of the point of intersection cc of the line of sight IE with the horizontal plane PH (containing the mosaic) when the angle 8] varies, for example, due to the rotation of the satellite around its axis U 1, U2. (When the mosaic is horizontal, this plane PH is coincident with the focal plane of the lens). It is seen that this locus line, which makes a constant angle with the axis U 1, U2 describes a cone when Bj varies. For Bj=0, point e0 is coincident with the focus F and, for Bj=90, this point goes to infinity and hence is rejected. It can be easily calculated that the locus of the point e0 is an hyperbola of vertex F but, as Bj is limited to i45, the point 20 is always a real point. For different values of the angle Ax, there is obtained a group of hyperbolas and, since the hyperbola formed when Ax=0 is coincident with the axis F Y and the hyperbolas formed when Ax90 is coincident with the axis FX, the radius of curvature of these hyperbolas, where Bj is constant, increases with Ax.

Because of the rotation of the satellite around the axis U l U2, this measured value Bj is not the exact value By of the sight angle and as discussed hereinabove By=BjBo.

For a given angle Bj, the satellite is considered as motionless and the locus of the point 20, on the plane PH, is a straight line parallel to the axis FX. For different angles Bj, the different straight lines obtained by variations of the value of the angle Ax are more and more spaced when Bj increases from a zero value.

Since the angle Ax is the complement of the angle between the line of sight 1e and the axis U 1 U2 of the satellite, the rotation of the satellite does not alter the value of said angle.

FIG. 6 represents the mosaic with its two families of curves, each case being constituted by a quadrilateral representing an equal value of angle (Ax)i and (Bjli of the two families of curves have been correctly set up.

Now the polar coordinates Ax, Bj, H are going to be trans formed into the cartesian system IX 1, IYl, [Z], the origin of which is the axis I of the satellite, and B] will be replaced by B =Bj-B0 in order to eliminate the effect of the rotation of the satellite with regard to the vertical of the place the lightning occurred,

The direction cosines a1, a2, b3 on the three axis of this new coordinate system are shown in thick lines on FIG. 5 in which the angles Ax and Bj are negative and their values are:

on the axis 1X1: 0 l =sin Ax on the axis IYl: a 2=cos Ax X sin By on the axis ill: a 3 cos Ax cos By These two last values have been obtained by, first, projecting the direction cosine lined up with line of sight Ie on the line Ie of the plane P0 (FIG. 5) and, second, by projecting this point on the axis Iyl and [Z].

In the cartesian coordinate system 0X 2, 0Y2, 022 shown in Fig. 4, having its origin at the center of the earth 0, the

direction cosines of the line of sight on the axises are bl, b2, b3. The direction cosines bl b2, b3 of the coordinate system X2, 0Y2, 0Z2 are deduced from the direction cosines a1, a2, a3 of the coordinate system IX 1, IX 1, 1Z1 by a simple translation along the vertical [0 with an amplitude of IO which results in The point E is defined by the intersection of the line of sight with the earth surface. If R is the length of the earth radius and H is the height of the satellite, the equation of the line of sight in the coordinate system 0X 2, 0Y2, 022 is:

There now will be a transformation into the geocentric coordinate system p, 0, d), in which the equation of the sphere is p---=R and the angles 0 and d) are defined in FIG. 4, it being understood that the values given to these represented angles are, respectively, negative and positive.

If the unit vector W is considered to be lined up with the radius OE, its projection QC on the horizontal plane X 2OY2 is p sin 6 and there is obtained:

.t=p sin cos 6 y=p sin 4) sin 6 Pp cos d) with p=R The two first terms of the equation (2) give: x/y=bl/b2 or:

By substituting for sin 0 its value obtained from equation (3) there is obtained an equation d =f y) that enables the calculation of the value of the angle From the values 0 and 4: of the geocentric coordinates of the point E this point can be localized, either by cartographic coordinates corresponding to a given projection system, or by geographic coordinates constituted, for example, by the latitude and the longitude. By way of a nonlimitative example, the localization system chosen in the present description enables the definition of the exact geographic coordinates of the point E in the following way.

From the point E there is drawn a great circle CE orthogonal to the great circle CS (FIG. 4) which intersects this circle atE'. The coordinates are then:

The ordinate Y3 is measured from the subsatellite point Sj whereas it has been seen previously that the ordinate of the point E is taken with respect to the origin point S0. The distance Y4==5Tj being the distance covered by the satellite in the time used to pass between the vertical of points SO and S], the value of 3 0? is easily calculated and is:

On the other hand, the earth zone where the greatest number of the thunderstorms occur is the equatorial zone that can be surveyed by a satellite with an equatorial or quasiequatorial orbit (maximum inclination 10) for which the earth rotation practically occurs in the orbit plane. It is enough, in this case, to take into account the earth rotation and to correct the value of Y4 by a constant factor.

However, if it is desired to look over other zones of the surface of the earth with a satellite having an inclination greater than 10, the values of 0 and 4) given by the equations (4) and (5) must also be corrected.

To determine the exact geographic coordinates Y3 and X, the right spherical triangle ESjE' is used. In this triangle we hagq On the other hand, it is seen in FIG. d, that the plane of the angle S/OC contains the great circle CD and that the plane of the angle .SJ'OYZ contains the great circle CS, so that the angle S] of the spheric triangle has the value Sj=(1r/2 )0.

The formula of spherical triangles enable the calculation of the coordinates X and Y3:

X=R arc sin (cos 0 sin zb) Y3=R arc tan (sin 6 tan (b).

It is seen that X and Y3 are each functions of 6 and d. which are themselves functions of Ax and By according to the equations (4) and (5).

Therefore, by calculation, tables can be set up that will give directly their values, which are:

X=Fl (Ax, By) a Y3=2 (Ax, By) (7) These coordinates are quantized in order to give the distances in number of quadrilaterals with respect to the origin quadrilateral centered on the point S0 (FIG. 4) and the distance Y4 is expressed in the same quantized unit.

The calculation of the coordinates X and Y, from the angles Ax, B], B0 measured by a mosaic, comprises only the following operations:

1. Calculation of the angle By=Bj-B0;

2. Calculation of the coordinates X and Y3 with the help of two-entry tables containing the functions symbolized by the equations (5) and (6).

3. Calculation of Y=Y3Y4.

The calculation itself, therefore, comprises the execution of two subtractions and of a search in a couple of two-entry tables receiving, on their inputs, the same parameters.

Because of the symmetry of FIG. 4 in the four quadrants formed by the intersection of the planes PO and PM all the values of each angle Ax and By are given, in the tables, by only n+l/2 inputs, the sign of X and Y3 being given by one of the angles Ax and By.

The value of the ordinate I obtained by this calculation can be greater than nl/2 which means that the point E can be situated in one of the zones contiguous to that centered on the point SO. Therefore, for obtaining, in a zone, the exact ordinate value, when Y n-l/2 (logical condition S FIG. 14), there must be made the operation Y0=Y(n-l/2) which will be called the ordinate correction. Moreover, as the parameters of the system have been chosen so that the mosaic covers on the ground a surface greater than that of a zone, it must be verified if Ya nl/2 and, in the contrary case (logical condition V FIG. 14), this light discharge is discarded.

The subtraction and comparison operations are made, in a well-known way, in the subtraction circuit that also performs the operations I and 3 hereabove mentioned.

FIG. 7 is a block diagram of the circuits that are on board the satellite for localizing the light phenomena occurring in the troposphere and comprises:

The block PS of electric mosaics comprising the mosaics PS1 to PS4;

The circuit ND for measuring and encoding the sight angle value;

The computer GC for calculating the geographic coordinates;

The discharge counting circuit DC;

The group of zone memories MZ;

The standard clock unit CU receiving signals from oscillator OS and from horizon sensor HS;

The high stability clock CS delivering the distance code Cy2 as well as the code CZ, the signal U0 and the zone memory selection signals :1, t2, :3 which will be defined when describing FIG. 13 and receiving the signal S from circuit DC;

The transmitter TR which transmits to the ground regularly spaced information; and

The receiver RV that receives, at regular intervals,

synchronizing information for clock CS.

The standard clock CU delivers, under the control of the oscillator OS, subcycle time signals ml to ml28 defining cycles lasting 4 milliseconds, each signal thus having a length of 3 l .2p.sec.

Each subcycle signal is divided into eight basic time slots cl to c8 which control the operations in the circuits GC and DC. As it is well known, the basic time slot signals are directly obtained from the signals delivered by the oscillator OS. The subcycle signals are obtained by applying, to a counter, an advance signal at each cycle of basic signals and this counter delivers the codes Cml to Cml 28 that are then decoded to obtain the signals ml to mi28.

As it is represented on the diagrams of FIG. 8, the subcycle signals are grouped in order to establish:

The signals IA and IB (FIG. 8b) used for measuring and calculating the coordinates;

The signals IC to IG (FIG. 8c) used for discharge counting and the ordinate correction.

The Table I hereunder shows the durations of each of these signals and their use.

TABLE I Time Duration Operations tA ml to m126 Measure of the angles by the block 01 mosaics PS, and their encoding in the circuit ND.

tB 111127 and n1l28 Csgcilation oi the coordinates in the circuit tC ml to 111100 Cyclic reading of the lines 1 to 100 of tho memory DC reserved to the lightnings being processed.

t1) mlOl Writing of the codes of a first discharge in one of the lines 1 to 100 of the memory DC.

tE m102 to n1106 Cyclic read and write for the lines 102 to 106 of the memory DC reserved for ordinate correction.

tl mm? to 111109 Execution oi an ordinate correction in the circuit GC.

tG 111127 Transfer oi the exact coordinates of alightning into the memory MZ and clearing.

FIG. 9 is a detailed block diagram of mosaics PS and circuit ND. The block of mosaics P5 is organized into a matrix of n columns (abscissae xi to xn) and b=4n lines (ordinates yl to yb), n being an odd number. When the cell placed at the intersection of the column 1' and the line j is excited, a current passes through it and circulates into the associated conductors xi, yj. Each of these conductors is connected, in the circuit ND (FIG. 7), to an amplifier having a passband of 250 l-Iz.25 kHz. The output signals of said amplifier control, at the time IA, when they are above a given threshold, the setting in I state of the first flip-flop in, respectively, the lateral sight angle register Gx (angle Ax) comprising n flip-flops, and the direct sight angle register Gy (angle By") comprising b flip-flops. This information is then translated into binary code by coders CRA and CRE which deliver codes CAx and CBj of the two sight angles.

In addition, horizon sensor HS (FIG. 7) delivers a pulse every time its line of sight is tangential to the earth surface. This information is processed in a well-known manner, and it gives the code CB0 of the position of a reference point on the surface of the satellite with respect to the vertical.

The codes CAx, CBj and CB0 are applied, at time IE, to the computer GC which comprises the circuits enabling the different operations hereabove described to obtain the exact geographic coordinates, the codes of which are CX and CY (or C Y0). As it has been seen these operations are subtractions and searches in the tables that contain the functions symbolized by the equations (6) and (7).

Computer GC is an arithmetic computer of well-known design and will not he described in detail. The speed of execution of the operations in this circuit depends on the type of circults used and on their dissipation. It is understood, however, that, if all the operations for obtaining the codes CX and CY cannot be made during two subcycle times as is indicated in the Table I (IB=ml27 and ml28), the duration of IE can be increased by reducing that of IA, or by reserving the times ml to ml06 and ml 10 to mi26 of the next cycle to the time IB. The time period m l 07 to m l 09 will, however, remain reserved to the ordinate correction (time IF).

At the end of time IB, computer GC delivers the codes CX and CY of the coordinates, with respect to the origin point S 0 (FIG. 4), of the quadrilateral in which the observed phenomenon has occurred. These codes are then transferred to circuit DC for storage in memory MC (FIG. Ill). FIG. 10 illustrates the detailed organization of memory MC which is divided horizontally into two address zones:

Zone A (addresses 1 to I00): Discharge memory for measuring the duration of the lightning, and

Zone B (addresses 101 to I06): Ordinate correction buffer.

Memory MC is also divided vertically into two parts:

Memory MXY for storing location codes CX, Cl, and

Memory Mt for storing elapsed time codes in its zone A and lightning duration data in its zone B.

The addresses 1 to 106 are selected under the control of signals delivered by decoder 08. This selection is made synchronously or asynchronously under the control of codes applied to decoder US as will be explained hereinafter during the description of FIG. 11.

Table II describes the different operations which are performed, once per cycle of 4 msec. and at times IC, ID, IE by means of these synchronous and asynchronous selections in both zones of memory MC.

TABLE II Memory MXY Time Memory Mt Synchronous readout 'Iho non-tree lines contain The free addresses the codes CX-CY of discontain acode zero. charge in course oi proe- The non-free ad essing. They are comdresses contain an pared to new discharges elapsed time code codes sent by GC. The Ct. codes are rewritten except This code is inii' the comparison yields: creased by one unit (a) A signal E characand rewritten exterizing alightning. cept it a signal E (b) A signal J characor J appears.

terizing a new discharge.

Zone A tD Asynchronous writing, in a Zero time code.

free address and under the control of a signal J, of the codes CX, CY of a new discharge.

tC Asynchronous Writing in a free address under the control of a signal Writing of the codes CX, Writing of the informaticn CY of a lightning.

Be or En characteriztng the duration (short or long) of this lightning.

Controlled synchronous readout which is stopped when a non-tree address is selected Codes CX, CY transferred Duration of lightning to registers RSX, RSY. appearing as a signal Be or Rn delivered by decoder DM.

Zone 13 At time IF, the code CY stored in register RSY (FIG. 11) is sent to computer CC for performing the ordinate correction and the code Cl is transferred into register REX (FIG. 11). The corrected code is sent back to circuit DC at the end of time IF and stored in register REY (FIG. 11).

Lastly, at time t6, the exact coordinates which are stored in registers REX and REY (FIG. II) are sent, together with the 

1. A thunderstorm observation satellite for determining the location of lightning comprising on board said satellite; first means to detect said lightning and provide quantized information about the value of two angles to enable determining unambiguously the position of said detected lightning projected on the earth surface; second means coupled to said first means to process and store said quantized information; and third means coupled to said second means to transmit said stored quantized information to ground stations at regular intervals; said second means including fourth means to determine and store the duration of said detected lightning.
 2. A satellite according to claim 1, wherein said fourth means includes a first cyclic memory means to store for a first given time the quantized position of one said lightning, fifth means coupled to said first memory means to compare the quantized position of said one said lightning to the quantized positions of all said lightning stored in said first memory means to produce an identity signal if said one said lightning has the same quantized position as that of one of said all said lightning, and sixth means coupled to said fifth means to generate a first signal if said identity signal has a first given delay relative to said first given time and a second signal if said identity signal has a second given delay different than said first given delay relative to said first given time.
 3. A satellite according to claim 2, wherein said second means further includes seventh means for processing said information concerning said lightning when one of said first and second signal have been generated having second cyclic memory means coupled to said first cyclic memory means, eighth means coupled to said second memory means to control asynchronously the readout of said information therefrom, ninth means coupled to said second memory means and said eighth means to block said readout as soon as a set of said information has been extracted, and tenth means coupled to said second memory means to produce a first control signal when said lightning occurs in a zone on said earth surface over which said satellite is located and a second control signal when said lightning occurs in the next adjacent zone on said earth surface.
 4. A satellite according to claim 3, wherein said first means defines on the earth surface a plurality of zones each of which are divided into n X n quadrilaterals, where n is an integer greater than one; and said second means further includes a plurality of zone memories each corresponding to a different one of said zones on said earth surface selected in time sequence according to the displacement of said satellite; each of said memories including n X n addresses reserved to the n X n quadrilaterals of one of said zone on said earth surface, and an additional address for storing the number of said zones which said satellite has flown over from a given reference point on said earth surface, each of said n X n addresses including one part to store the number of said first signal and second part to store the number of said second signal.
 5. A satellite according to claim 4, further including eleventh means to transfer said information from said seventh means to the appropriate one of said zone memories and for selecting the appropriate address in said one of said zone memories when said first control signal is present and in the next one of said zone memories when said second control signal is present.
 6. A satellite according to claim 5, further including twelfth means coupled to said eleventh means to increase by one unit the number stored in said one part of said appropriate address if said first signal is present and to increase by one unit the number stored in said second part of said appropriate address if said second signal is present.
 7. A satellite according to claim 6, wherein said first means include a plurality of n X n matrices of photosensitive means, where n is an integer greater than one. 